Process for the preparation of aplha-alumina powder

Abstract: A process for producing α-alumina powder comprises calcining transition alumina and/or a transition alumina precursor which transforms to transition alumina on heating, in a gas atmosphere containing (1) a hydrogen halide, (2) a component prepared from a halogen and steam or (3) a halogen, and in the presence of seed crystals and/or a shape-controlling agent. The product α-alumina powder comprises particles of a substantially octahedral or eicosa-hedral shape, a pre-determined structure, and a narrow primary particle size distribution; the powder is particularly useful as a raw material for preparing abrasives, fillers, sinters or spacers.

Description: This invention relates to a process for producing α-alumina powder having excellent characteristics and to a process for its production. α-Alumina powder is widely used as a raw material for abrasives, sinters, plasma spraying materials, fillers, etc. α-Alumina powder obtained by conventional processes that have been generally employed comprises irregular-shaped polycrystals, contains many agglomerated particles, and has broad particle size distribution. The purity of such conventional α-alumina products is insufficient for some uses. In order to avoid these disadvantages and, for some specific uses, to control primary particle size or shape, α-alumina powder produced by special processes as hereinafter described has been employed. However, these special processes still involve difficulty in producing α-alumina powder with a narrow primary particle size distribution, which comprises α-alumina particles having a controlled shape, a controlled primary size, and homogeneity. Further, in order to orientate α-alumina particles in a specific direction when packed or laminated in layer(s), it is desirable that the powder comprises α-alumina particles having a rod-like shape, a cocoon shape or a thick plate shape. However, conventionally-obtained α-alumina powder has broad primary particle size distribution or the particles thereof have a thin plate shape, and it has hitherto been difficult to obtain α-alumina powder having a narrow primary particle size distribution and with the particles thereof having a shape suitable for orientation. Among general processes for producing α-alumina powder a Bayer's process is the most economical process. In a Bayer's process, bauxite is converted to aluminum hydroxide or transition alumina, which is then calcined in air to prepare α-alumina powder. The aluminum hydroxide or transition alumina which is obtained as an intermediate product on an industrial scale at low cost comprises agglomerated particles having a diameter of greater than 10 µm. α-Alumina powder obtained by calcination of such aluminum hydroxide or transition alumina in air comprises primary particles of irregular shape containing many strongly agglomerated coarse particles. The α-alumina powder containing coarse agglomerated particles is ground into a final product by means of a ball mill, a vibration mill, etc., but grinding is not always easy and increases the cost. Further, α-alumina powder having poor grindability needs a long grinding time; consequently too fine a powder may be formed. Several proposals have been made to solve these problems. For example, JP-A-59-97528 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") teaches a process for improving the shape of α-alumina powder, which comprises calcining aluminum hydroxide prepared by a Bayer process in the presence of an ammonium-containing boron or boron series mineralizer to obtain α-alumina powder having an average primary particle diameter of from 1 to 10 µm and a D/H ratio approximate to 1, wherein D is the maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of α-alumina, and H represents a maximum particle diameter perpendicular to the hexagonal lattice plane. Since the starting aluminum hydroxide has a particle diameter of several tens of micrometers (µm) or greater, and the calcination is carried out in a rotary kiln, the resulting α-alumina powder has a broad primary particle size distribution and the particles thereof have irregular shapes. It is also difficult to control the primary particle size or shape. Known special processes for producing α-alumina powder include a hydrothermal process utilizing a hydrothermal reaction of aluminum hydroxide; a flux process comprising adding a flux to aluminum hydroxide, fusing, and precipitating; and a process comprising calcination of aluminum hydroxide in the presence of a mineralizer. With respect to a hydrothermal process, JP-B-57-22886 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses addition of corundum as a seed crystal to control the particle size. Because the synthesis in this process is carried out at a high temperature and under high pressure, the resulting α-alumina powder is expensive to produce. WO-A-9015777 discloses a process for producing α-alumina particles having a plate-like shape, in which aluminum hydrate is obtained by using seed crystals, and the aluminum hydrate is calcined in the presence of a mineralizer (chlorine or hydrochloric acid). Only α-alumina particles having a plate-like shape can be obtained by this process, and there is no description about the process of obtaining α-alumina particles having various controlled shape. DE-A-2850064 discloses a process for producing α-alumina particles having a hexagonal shape, in which aluminum hydroxide, to which α-alumina is added as seed crystals, is calcined in the presence of a fluorine-containing compound. Only α-alumina particles having a hexagonal shape can be obtained by this process, and there is no description about the process of obtaining α-alumina particles having various controlled shape. A flux process has been proposed as a means for controlling the particle shape or primary particle size of α-alumina powder for use as e.g. an abrasive or a filler. For example, JP-B-3-131517 discloses a process comprising calcining aluminum hydroxide in the presence of a fluorine series flux having a melting point of not more than 800°C to prepare α-alumina powder which comprises hexagonal plate-shaped a-alumina particles having an average primary particle size of from 2 to 20 µm and a D/H ratio of from 5 to 40, wherein D and H are as defined above. However, this process cannot provide fine α-alumina powder having a primary particle diameter of 2 µm or less, and all the particles obtained have a plate shape. In other words, the shape and particle size cannot be controlled as desired by this process. Journal of American Ceramic Society, Vol. 68, No. 9, pp. 500-505 (1985) reports that the temperature of α transition can be reduced by addition of α-alumina to boehmite. However, since the purpose thereof is to obtain a sintered body of fine grain size, α-alumina powder with controlled primary particle size and shape cannot be obtained by this technique. WO-A-9324681, a document falling within the terms of Article 54(3) EPC., discloses a process for producing α-alumina having no seed crystal inside and having a number average particle diameter of more than 5 µm and not more than 30 µm, in which transition alumina is calcined under the same atmosphere as in the present invention. However, there is no description of the use of a seed crystal and a shape controlling agent. U.S. Patent 4,657,754 discloses a process for obtaining α-alumina powder whose particles are smaller than 1 µm in diameter comprising adding α-alumina seed crystals to an α-alumina precursor, followed by calcination and grinding. The powder obtained by the above calcination comprises agglomerates of fine primary particles of not greater than 1 µm, and primary particles greater than 10 µm cannot be obtained by the process. Therefore, there has not yet been established a technique for producing α-alumina powder in which the primary particle diameter can be controlled from submicrons to several tens of microns; the particle shape can be controlled from a hexagonal plate shape to a column shape; or the particle size distribution can be narrowed while controlling the crystal habit of the a face {11 2 0}, c face {0001}, n face {22 4 3} and r face {10 1 2}. There has been a keen demand to develop such techniques. Moreover, there has not yet been obtained α-alumina powder whose particles have a sufficient thickness for easy orientation; which has a narrow primary particle size distribution; and which is particularly suitable as a raw material for abrasives, fillers and sinters or as a spacer. An object of the present invention is to provide a process for producing α-alumina powder having a narrow primary particle size distribution, and in which the size and shape of the primary particles can be controlled in a pre-determined manner. A further object of the present invention is to provide α-alumina powder comprising α-alumina particles having a substantially octahedral or eicosahedral shape, a specific structure, and a narrow primary particle size distribution. Such powder is the most useful as a raw material for abrasives, fillers and sinters. We have found that the above objects of the present invention are attained by calcining a raw material, such as transition alumina, in a specific gas atmosphere in the presence of a seed crystal and a shape-controlling agent. According to the present invention there is provided a process for producing α-alumina powder which comprises α-alumina particles having an octahedral or higher polyhedral shape and a hexagonal close-packed lattice having a D/H ratio of from 0.5 to 30, wherein D represents the maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of α-alumina, and H represents the maximum particle diameter perpendicular to the hexagonal lattice plane; comprising the step of calcining in a gas atmosphere selected from (1) a gas atmosphere containing a hydrogen halide, (2) a gas atmosphere containing a component prepared from a halogen and steam, and (3) a gas atmosphere containing a halogen, at least one of transition alumina and a transition alumina precursor capable of being transformed to transition alumina on heating, characterized in that the calcination occurs in the presence of seed crystals and a shape-controlling agent. In the accompanying drawings FIG. 1 is a scanning electron microscope (SEM) photograph (magnification: 9000) showing the particulate structure of the α-alumina powder obtained in Reference Example 1. FIG. 2 is an SEM photograph (magnification: 900) showing the particulate structure of the α-alumina powder obtained in Reference Example 2. FIG. 3 is an SEM photograph (magnification: 10000) showing the particulate structure of the α-alumina powder obtained in Reference Example 19. FIG. 4 shows primary particle size distribution of the α-alumina powder obtained in Reference Example 19. FIG. 5 is an SEM photograph (magnification: 1000) showing the particulate structure of the α-alumina powder obtained in Reference Example 33. FIG. 6 shows primary particle size distribution of the α-alumina powder obtained in Reference Example 33. FIG. 7 is an SEM photograph (magnification: 9000) showing the particulate structure of the α-alumina powder obtained in Reference Example 53. FIG. 8 is an SEM photograph (magnification: 900) showing the particulate structure of the α-alumina powder obtained in Reference Example 54. FIG. 9 is an SEM photograph (magnification: 9000) showing the particulate structure of the α-alumina powder obtained in Reference Example 57. FIG. 10 is an SEM photograph (magnification: 4300) showing the particulate structure of the α-alumina powder obtained in Example 3. FIG. 11 is an SEM photograph (magnification: 9000) showing the particulate structure of the α-alumina powder obtained in Reference Example 60. FIG. 12 is an SEM photograph (magnification: 1710) showing the particulate structure of the α-alumina powder obtained in Reference Example 63. FIG. 13 is an SEM photograph (magnification: 1280) showing the particulate structure of the α-alumina powder obtained in Reference Example 64. FIG. 14 is an SEM photograph (magnification: 10000) showing the particulate structure of the α-alumina powder obtained in Comparative Example 1. FIG. 15 shows primary particle size distribution of the α-alumina powder obtained in Comparative Example 1. FIG. 16 shows the crystal habit of α-alumina particles. According to the process of the present invention, the α-alumina powder can be prepared from transition alumina and/or a raw material capable of being converted to transition alumina on heating (hereinafter referred to as a transition alumina precursor). Transition alumina means all alumina species included under polymorphic alumina represented by Aℓ 2 O 3 except α-alumina. Specific examples of transition alumina including γ-alumina, δ-alumina, and -alumina. The transition alumina precursor includes those that can be converted to α-alumina via transition alumina by calcination. Examples of the transition alumina precursor include aluminum hydroxide, aluminum sulfate, alum (e.g., potassium aluminum sulfate or ammonium aluminum sulfate), ammonium aluminum carbonate, and alumina gel (e.g., one obtained by electro-discharge in water). The transition alumina and transition alumina precursors which can be used in the present invention are not particularly limited by the synthetic method; eg, aluminum hydroxide can be obtained by a Bayer process, hydrolysis of an organoaluminum compound, or a process starting with an aluminum compound recovered from an etching waste used for condensers, etc., and transition alumina can be obtained by a heat treatment of aluminum hydroxide, decomposition of aluminum sulfate, decomposition of alum, gaseous phase decomposition of aluminum chloride, or decomposition of ammonium aluminum carbonate. According to the process of the present invention, desired α-alumina powder can be obtained even from aluminum hydroxide or transition alumina having a particle size of 10 µm or greater which is obtained by an industrially economical process, such as a Bayer process. The seed crystals used in the present invention are crystals serving as nuclei for the crystal growth of α-alumina. A crystal of α-alumina grows around each seed crystal. Any seed crystal may be used as long as this function is performed. Preferred seed crystals include compounds of aluminum, titanium, vanadium, chromium, iron or nickel and mixtures thereof. The compounds of these metals include oxides, nitrides, oxinitrides, carbides, carbide nitrides, and borides, with oxides and nitrides being preferred. Compounds of vanadium can serve both as the seed crystal and as a shape-controlling agent. Proper selection of the kind and the amount of the seed crystal makes it possible to produce α-alumina powder comprising α-alumina particles having pre-determined primary particle size and a specific polyhedral shape. The amount of the seed crystal is usually from 10 ppm to 50 parts by weight, preferably from 100 ppm to 30 parts by weight, more preferably from 200 ppm to 10 parts by weight, per 100 parts by weight of transition alumina to be used or to be converted from its precursor. The primary particle size of α-alumina obtained can be controlled by the number of seed crystals added. The greater the number of seed crystals added, the smaller is the size of α-alumina particles obtained. The shape-controlling agent used in the present invention is an agent that, through some unknown mechanism, acts on crystal growth to change the D/H ratio as hereinafter described and the crystal habit. Preferred shape-controlling agents include metals, such as magnesium, calcium, strontium, yttrium, zirconium, vanadium, niobium, molybdenum, copper, zinc, boron, silicon, lanthanum, cerium, and neodymium, compounds of these metals, and mixtures thereof. The compounds of the metals include oxides, nitrides, oxinitrides, carbides, carbide nitrides, halides, and borides, with oxides being preferred. Proper selection of the shape-controlling agent makes it possible to produce α-alumina which comprises particle powder having a primary particle size and shape controlled to suit the end use. The amount of the shape controlling agent is usually from 10 ppm to 50 parts by weight, preferably from 100 ppm to 30 parts by weight, more preferably 200 ppm to 10 parts by weight, per 100 parts by weight of transition alumina to be used or to be converted from its precursor. In the case of controlling the D/H ratio of α-alumina particles, for example, α-alumina particles having a greater D/H ratio can be obtained in proportion to increase of the amount of the agent for increasing D/H ratio. In the case of controlling crystal habit of α-alumina particles, for example, the area of N-plane in α-alumina particle becomes larger in proportion to increase of the amount of the agent for forming N-plane. Both the seed crystal and the shape-controlling agent are used in combination whereby α-alumina powder which comprises α-alumina particles having a controlled primary particle size, a controlled D/H ratio, controlled crystal habit, and a controlled shape is obtained. The total amount of the seed crystal and the shape controlling agent is usually from 10 ppm to 50 parts by weight, preferably 100 ppm to 30 parts by weight, more preferably from 200 ppm to 10 parts by weight, per 100 parts by weight of transition alumina to be used or to be converted from its precursor. The present invention is generally carried out by mixing the above-mentioned materials (i.e., a transition alumina and/or a precursor thereof, and a seed crystal and a shape controlling agent), and calcining the mixture. The method of mixing is not particularly restricted. Where a wet mixing method is followed, for example, either an aqueous solvent or an organic solvent may be used. Mixing can be conducted by means of a ball mill or a vertical granulator. Ultrasonification and stirring may also be employed. Worn substance of material from a mixing device, such as in a mixing medium, may be used as a seed crystal or a shape-controlling agent. For example, α-alumina waste generated from α-alumina balls during ball mill mixing may be used as a seed crystal. The transition alumina and/or a precursor thereof mixed with the seed crystal and a shape-controlling agent is calcined in (1) a gas atmosphere containing a hydrogen halide preferably in a concentration of 0.1% by volume or more, more preferably 0.5% by volume or more, and most preferably 1.0% by volume or more. Component(s) other than hydrogen halide in the gas atmosphere, namely diluent gases include inert gases, e.g., nitrogen, hydrogen and argon, and air. The pressure of the gas atmosphere is not critical and may be selected at will from an industrially practical range. α-Alumina powder having excellent properties as desired can be obtained by calcination at a relatively low temperature as hereinafter described. The gas atmosphere containing hydrogen halide (1) may be replaced with (2) a gas atmosphere containing a component prepared from a halogen and steam. In this case, transition alumina and/or a precursor thereof is calcined in an atmosphere containing the component prepared from a halogen preferably in a concentration of 0.1% by volume or more, more preferably 0.5% by volume or more, and most preferably 1.0% by volume or more, and steam preferably in a concentration of 0.01% by volume or more, more preferably 0.1% by volume or more, and most preferably 0.5% by volume or more. The component prepared from a halogen and steam can be obtained in a known manner, for example, by introducing a halogen and steam into the system: or a halogen and steam may previously be mixed and then introduced to the system. Component(s) other than the component prepared from the halogen and steam in the gas atmosphere, namely diluent gases, include inert gases, e.g., nitrogen, hydrogen, and argon, and air. The pressure of the atmosphere is not particularly critical and may be selected at will from an industrially practical range. α-Alumina powder having excellent properties as desired can be obtained by calcination at a relatively low temperature as hereinafter described. When the calcining is conducted in a gas atmosphere containing a hydrogen halide (1) or a gas atmosphere containing a component prepared from a halogen and steam, the calcining temperature is preferably from 500° to 1400°C, more preferably from 600° to 1300°C, and most preferably from 700° and 1200°C. By calcining at a temperature controlled within this range, α-alumina powder comprising α-alumina particles that are hardly agglomerated and, even immediately after calcination, show a narrow primary size distribution can be obtained at an industrially advantageous rate of formation. Where the starting material, i.e., transition alumina and/or a precursor thereof, has a large particle size, for example, where agglomerated particles having an average particle size exceeding 10 µm are used, a relatively high calcining temperature within the above-recited range, particularly 700°C or higher is preferred. The gas atmosphere containing hydrogen halide (1) may also be replaced by (3) a gas atmosphere containing a halogen. In this case, transition alumina and/or a precursor thereof is calcined in an atmosphere containing a halogen preferably in a concentration of at least 0.1% by volume, more preferably 0.5% by volume or more, and most preferably 1.0% by volume or more. Component(s) other than a halogen in the gas atmosphere, namely diluent gases, include inert gases, e.g., nitrogen, hydrogen, and argon, and air. The pressure of the atmosphere is not critical and may be selected at will from an industrially practical range. α-Alumina powder having excellent properties as desired can be obtained. Examples of the halogen which can be used in the present invention includes fluorine, chlorine, bromine, and iodine, with fluorine and chlorine being preferred, and chlorine being more preferred. Where the gas atmosphere containing a halogen (3) is used, the calcining temperature is preferably from 950° to 1500°C, more preferably from 1050° to 1400°C, and most preferably from 1100° and 1300°C. By calcining at a temperature controlled within this range, α-alumina powder comprising α-alumina particles that are hardly agglomerated and, even immediately after calcination, show a narrow size distribution, can be obtained at an industrially advantageous rate of formation. Where the starting material has a large particle size, for example, where agglomerated particles having an average particle size exceeding 10 µm are used, a relatively high calcining temperature within the above-recited range, particularly 1100°C or higher is preferred. Calcination of transition alumina and/or a precursor thereof should be continued for a time sufficient for the starting materials to grow to α-alumina. While depending on the concentration of the gas atmosphere, the calcining temperature, and the like conditions, the calcination time is generally 1 minute or more, and preferably 10 minutes or more, but not limited thereto. The source of gas and the method of supplying gas to the system are not particularly restricted, as long as the above-mentioned gas atmosphere is fed to the reaction system containing the starting materials. For example, a cylinder containing the gas is usually used as a supply source. Where an aqueous solution of a hydrogen halide, a halogen compound, e.g., an ammonium halide, or a halogen-containing high polymer is used, it can be fed at its vapor pressure or decomposed so as to give a prescribed gas composition. The gas supply may be either in a continuous manner or in a batch system. The calcining apparatus is not particularly limited, and a general calcining furnace can be employed. The calcining furnace is preferably made of a material resistant to corrosion by a hydrogen halide gas, a halogen gas, etc. The furnace is preferably equipped with a mechanism for controlling the atmosphere. Because an acidic gas, e.g., a hydrogen halide or a halogen gas, is used, the furnace is preferably air-tight. For industrial production, calcination is preferably carried out in a continuous manner by means of, for example, a tunnel kiln, a rotary kiln or a pusher oven. Since the reaction proceeds in an acidic gas atmosphere, a crucible, a boat or a like tool used in the process is preferably made of alumina, quartz, acid resistant brick or graphite. According to the process of the present invention, α-alumina powder can be obtained that comprises α-alumina particles having an octahedral or higher polyhedral shape as shown in FIG. 16, a hexagonal close-packed lattice having a D/H ratio of from 0.5 to 30, and homogeneity. The α-alumina particles show narrow primary particle size distribution and usually have an average primary particle diameter of from 0.1 to 30 µm and a D/H ratio of from 0.5 to 30. In particular, the present invention provides α-alumina powder comprising α-alumina particles having a substantially octahedral or eicosahedral shape, a hexagonal close-packed lattice having a D/H ratio of from 0.5 to 30, rotational symmetry of a symmetry number of 6 on the axis perpendicular to the lattice plane, and a size distribution of not more than 10, preferably not more than 5, as expressed in terms of D 90 /D 10 , wherein D 10 and D 90 represent a cumulative 10% diameter and a cumulative 90% diameter, respectively, of a cumulative distribution depicted from the small diameter side. α-Alumina powder having such a narrow primary particle size distribution is particularly suitable as a raw material for abrasives, fillers, spacers, and sinters. The characteristics whereby an indistinguishable figure appears for every rotation by 2π/n (n is a positive integer) is designated rotational symmetry; n is designated a symmetry number; and the axis of rotation is designated n-fold axis of symmetry. According to the process of the present invention, α-alumina powder which comprises α-alumina particles having homogeneity, narrow primary particle size distribution, and an octahedral or higher polyhedral shape, can be obtained from alumina raw materials of a variety of kinds, shapes, sizes, and compositions. Appropriate selection of the kind and amount of seed crystal makes it possible to obtain α-alumina powder comprising α-alumina particles having a pre-determined primary particle size and a specific polyhedral shape. Since the D/H ratio and crystal habit can be varied by the use of a shape-controlling agent, appropriate selection of shape-controlling agent makes it feasible to prepare α-alumina powder comprising α-alumina particles having any shape suited to the end use. The combined use of seed crystals and a shape-controlling agent makes it possible to produce α-alumina powder which comprises α-alumina particles having a predetermined primary particle size, a pre-determined D/H ratio, a crystal habit, and a selected shape suited to the end use. The α-alumina powder obtained by the process of the present invention has excellent characteristics such as an average particle size of from 0.1 to 30 µm, a D/H ratio of the particles of from 0.5 to 30, and a narrow primary particle size distribution. The α-alumina powder obtained by the process of the present invention has a narrow primary particle size distribution with its primary particle diameter controlled at the level of several micrometers (µm) and is therefore suitable as a raw material for encapsulation. In addition to the above-mentioned characteristics, the α-alumina powder has a high bulk density and can therefore attain a high packing density as a raw material for forming single crystals. Further, since the primary particle size of the α-alumina powder can be controlled arbitrarily, the powder can be used as a raw material for ceramic filters having various pore sizes. Furthermore, the particles of the α-alumina powder can have their edge shape varied by controlling the crystal habit so as to have a controlled abrasion effect. The α-alumina powder obtained by the process of the present invention is suitable as a raw material for abrasives, sinters, plasma spraying materials, fillers, single crystals, supports for catalysts, fluorescent substances, encapsulant, and ceramic filters. In particular, the α-alumina powder substantially made up of octahedral or eicosahedral shape is the most suitable as a raw material for abrasives, fillers, sinters or spacers and is extremely useful in industry. The present invention will now be illustrated in greater detail with reference to Examples, but it should be understood that the present invention is not construed as being limited thereto. Measurements in Examples and Comparative Examples were made as follows: (1) Primary Particle Diameter and Primary Particle Size Distribution of α-alumina: A micrograph of α-alumina powder was taken with a scanning electron microscope (SEM) ("T-300" manufactured by JEOL Ltd., hereinafter the same), and 80 to 100 particles selected were subjected to image analysis to obtain an average and the distribution of the circle-equivalent diameters. The term "circle-equivalent diameter" as used herein means a diameter of a true circle having the same area as that of a particle. (2) Crystal Shape of α-alumina (D/H Ratio): The shape of α-alumina particles was expressed in terms of a D/H ratio, wherein D and H are as defined above. Five to 10 particles were selected from the above-mentioned SEM photograph and subjected to image analysis to obtain an average of the D/H ratios. (3) Crystal Habit: The crystal habit of α-alumina particles was observed for evaluating the shape. The crystal habit of the polyhedral α-alumina particles obtained in the present invention (indicated by A to I) is shown in FIG. 16. α-alumina has a hexagonal system, and the term "crystal habit" used for α-alumina means the form of its crystal characterized by the way of appearance of the crystal faces composed of a face {11 2 0}, c face {0001}, n face {22 4 3}, and r face {10 1 2}. In FIG. 16 are shown crystal faces a, c, n, and r. (4) Number of Crystal Faces: Determined by image analysis of the SEM photograph. (5) Rotational Symmetry: Determined by observation on the SEM photograph. (6) D 90 /D 10 : Determined with a Master Sizer (manufactured by Malbern Co.) which follows the principle of laser scattering. (7) Compressive Strength Compressive strength of particles was measured with dynamic ultra-micro hardness tester (DVH-200, manufactured by SHIMADZU CORPORATION). (8) Observation of Internal Micro Structure with TEM Internal micro structure of α-alumina particle was obseved with Ultra High Voltage Transmitte Electron Microscope (TEM) (acceleration voltage 1200 KVA, manufactured by HITACHI, LTD.). (9) Tapped Density Tapped density of α-alumina powder was measured according to JIS-H-1902. Raw materials to be calcined, seed crystals, and shape-controlling agents used in Reference examples, Examples and Comparative Examples are shown below. Raw Materials: 1. Transition alumina A: Transition alumina obtained by calcination of aluminum hydroxide prepared by hydrolysis of aluminum isopropoxide ("AKP-G15" produced by Sumitomo Chemical Co., Ltd.; particle diameter: about 4 µm) (abbreviated as tr-al A in Tables). 2. Transition alumina B: Transition alumina obtained by decomposition of alum ("CR 125" produced by Baikowski Chimie; particle diameter: about 4 µm) (abbreviated as tr-al B in Tables). 3. Aluminum hydroxide A: Powder prepared by hydrolysis of aluminum isopropoxide (secondary particle diameter: about 8 µm) (abbreviated as al-hr A in Tables). 4. Aluminum hydroxide B: Powder prepared by a Bayer process ("C 301" produced by Sumitomo Chemical Co., Ltd.; secondary particle diameter: about 4 µm) (abbreviated as al-hy B in Tables). 5. Aluminum hydroxide C: Powder prepared by a Bayer process ("C 12" produced by Sumitomo Chemical Co., Ltd.; secondary particle diameter: about 30 µm) (abbreviated as al-hy C in Tables). 6. Alum (AℓNH 4 (SO 4 )·12H 2 O): Transition alumina precursor which gives transition alumina on heating. The reagent of Wako Pure Chemical Industries, Ltd. was used. 7. Aluminum sulfate (Aℓ 2 (SO 4 ) 3 ·16H 2 O): Transition alumina precursor which becomes transition alumina on heating. A product of Sumitomo Chemical Co., Ltd. was used. Seed Crystals: 1. α-alumina A: "AKP-50", α-alumina powder produced by Sumitomo Chemical Co., Ltd.; average primary particle diameter: about 0.3 µm; abbreviated as α-al A in Tables. 2. α-alumina B: "AKP-15", α-alumina powder produced by Sumitomo Chemical Co., Ltd.; average primary particle diameter: about 0.8 µm; abbreviated as α-al B in Tables. 3. α-alumina C: α-alumina particles generated from alumina balls during ball milling (abbreviated as α-al C in Tables). 4. α-alumina D: α-alumina obtained by a method of the present invention; average primary particle diameter: about 3 µm. 5. Titanium oxide (TiO 2 ): A reagent produced by Fuji Titanium Industry Co., Ltd. 6. Chromium oxide (Cr 2 O 3 ): A reagent produced by Wako Pure Chemical Industries, Ltd. 7. Iron oxide (Fe 2 O 3 ): A reagent produced by Bayer Japan Ltd. 8. Nickel oxide (Ni 2 O 3 ): A reagent produced by Nakarai Kagaku Yakuhin K.K. 9. Vanadium oxide (V 2 O 5 ): A reagent produced by Nakarai Kagaku Yakuhin K.K. 10. Aluminum nitride (AℓN): A reagent produced by Tokuyama Soda Co., Ltd. Shape-Controlling Agent: 1. Magnesium oxide (MgO): A reagent produced by Wako Pure Chemical Industries, Ltd. 2. Boron oxide (B 2 O 3 ): A reagent produced by Wako Pure Chemical Industries, Ltd. 3. Magnesium hydroxide (Mg(OH) 2 ): A reagent produced by Wako Pure Chemical Industries, Ltd. 4. Silicon oxide (SiO 2 ): A reagent produced by Nippon Aerosil Co., Ltd. 5. Zirconium oxide (ZrO 2 ): Zirconia gel obtained by hydrolysis of zirconium oxychloride (ZrOCℓ 2 ) produced by Nakarai Kagaku Yakuhin K.K. 6. Copper oxide (CuO): A reagent produced by Nakarai Kagaku Yakuhin K.K. 7. Strontium oxide (SrO): A reagent produced by Nakarai Kagaku Yakuhin K.K. 8. Zinc oxide (ZnO): A reagent produced by Nakarai Kagaku Yakuhin K.K. 9. Molybdenum oxide (MoO 3 ): A reagent produced by Nakarai Kagaku Yakuhin K.K. 10. Niobium oxide (Nb 2 O 5 ): A reagent produced by Nakarai Kagaku Yakuhin K.K. 11. Calcium oxide (CaO): A reagent produced by Wako Pure Chemical Industries, Ltd. 12. Boron oxide (B 2 O 3 ): A reagent produced by Wako Pure Chemical Industries, Ltd. 13. Yttrium oxide (Y 2 O 3 ): A reagent produced by Nippon Yttrium K.K. 14. Lanthanum oxide (La 2 O 2 ): A reagent produced by Nakarai Kagaku Yakuhin K.K. 15. Cerium oxide (CeO 2 ): A reagent produced by Santoku Kinzoku Kogyo K.K. 16. Neodymium oxide (Nd 2 O 3 ): A reagent produced by Nippon Yttrium K.K. Mixing of materials was performed by ultrasonification using isopropyl alcohol as a solvent, except for milling using an alumina ball. Drying after mixing was carried out by means of a rotary evaporator and a drier. Hydrogen chloride packed in a cylinder produced by Tsurumi Soda K.K. (purity: 99.9%) was used as a hydrogen chloride gas source, and chlorine packed in a cylinder produced by Fujimoto Sangyo K.K. (purity: 99.4%) was used as a chlorine gas source. A decomposition gas of ammonium fluoride was used as a hydrogen fluoride gas source. Ammonium fluoride was heated to its subliming temperature, 220°C, and the resulting decomposition gas was introduced into the tube of a tubular furnace. Ammonium fluoride completely decomposed when kept at 1100°C to provide an atmosphere comprising 33% by volume of hydrogen fluoride, 17% by volume of hydrogen, and 50% by volume of nitrogen. A decomposition gas of ammonium bromide was used as a hydrogen bromide gas source. Ammonium bromide was heated to its subliming temperature, 420°C, and the resulting decomposition gas was introduced into the tube of a tubular furnace. Ammonium bromide completely decomposed when kept at 1100°C to provide an atmosphere comprising 33% by volume of hydrogen bromide, 17% by volume of hydrogen, and 50% by volume of nitrogen. A decomposition gas of ammonium iodide was used as a hydrogen iodide gas source. Ammonium iodide was heated to its subliming temperature, 380°C, and the resulting decomposition gas was introduced into the tube of a tubular furnace. Ammonium iodide completely decomposed when kept at 1100°C to provide an atmosphere comprising 33% by volume of hydrogen iodide, 17% by volume of hydrogen, and 50% by volume of nitrogen. A prescribed amount of seed crystal and shape-controlling agent was added to a starting material (transition alumina and/or a precursor thereof), and 0.4 g aliquot of the mixture was put' in an alumina boat to a height of 5 mm. Calcination was carried out in a tubular furnace ("DSPSH-28" produced by Motoyama K.K.) using a quartz-made core tube (diameter: 27 mm; length: 1000 mm). The temperature was raised at a rate of 500°C/hr while feeding nitrogen gas, and a gas having a prescribed composition was introduced into the tubular furnace when the temperature reached a prescribed temperature. The steam partial pressure was controlled by adjusting the saturated vapor pressure of water dependent on temperature, and the steam was introduced into a furnace with a nitrogen carrier gas. The gas concentration was controlled by adjustment of the gas flow rate by means of a flowmeter. The linear flow rate was set at 20 mm/sec (gas flow system). In Example 9 in which the hydrogen chloride concentration was low, the gas flow system was replaced with a system in which a prescribed gas atmosphere was introduced, and the gas feed was stopped to conduct calcination. In all Reference examples, Examples and Comparative Examples, calcination was conducted under atmospheric pressure. On reaching a prescribed temperature, the furnace was maintained at that temperature (hereinafter referred to as a calcining temperature) for a prescribed time (hereinafter referred to as a calcination time). After a lapse of a prescribed calcination time, the furnace was allowed to cool to obtain α-alumina powder. "Part(s) by weight" in all Examples are based on 100 parts by weight of transition alumina to be used or to be converted from its precursor. REFERENCE EXAMPLE 1 Transition aluminum A was mixed with 3 parts by weight of γ-alumina A as a seed crystal. Hydrogen fluoride gas obtained by decomposition of ammonium fluoride was fed at 800°C, and the mixture was calcined at 1100°C for 30 minutes. The calcining conditions and the results are shown in Tables 1 and 2. The SEM photograph of the α-alumina powder obtained is shown in FIG. 1. REFERENCE EXAMPLE 2 Calcination of transition aluminum A was carried out under the same conditions as in Reference Example 1, except for replacing α-alumina A as a seed crystal with 1 part by weight, based on the alumina obtained from transition alumina A, of zirconia gel (ZrO 2 ) as a shape-controlling agent. The calcination conditions and the results obtained are shown in Tables 1 and 2. The SEM photograph of the α-alumina powder obtained is shown in FIG.2. REFERENCE EXAMPLE 3 Calcination was carried out in the same manner as in Reference Example 1, except for replacing transition alumina A (γ-alumina) with aluminum hydroxide A as a raw material. The same α-alumina powder as obtained in Reference Example 1 was obtained. The calcination conditions are shown in Tables 1 and 2. REFERENCE EXAMPLE 4 Calcination of transition alumina A was carried out in the same manner as in Reference Example 1, except for replacing hydrogen fluoride with fluorine and steam. The α-alumina powder obtained was equal to that obtained in Reference Example 1. The calcination conditions are shown in Tables 1 and 2. REFERENCE EXAMPLES 5 TO 9 Aluminum hydroxide A to which 0.1 part by weight, based on the alumina obtained from aluminum hydroxide A, of α-alumina A had been added was calcined in an atmosphere having a varied hydrogen chloride gas concentration at 1100°C. The calcination time was varied in accordance with the hydrogen chloride concentration. Hydrogen chloride gas was fed at 800°C. The calcination conditions and the results obtained are shown in Tables 1 and 2. The internal structure of the α-alumina particles obtained in Reference Example 6 was observed with TEM. No defect in the particles was detected. REFERENCE EXAMPLES 10 AND 11 Aluminum hydroxide A was calcined in the same manner as in Reference Example 6, except for changing the temperature at which hydrogen chloride gas was introduced and the calcining temperature and time as shown in Table 1. The results obtained are shown in Table 2. REFERENCE EXAMPLE 12 Aluminum hydroxide A was calcined in the same manner as in Reference Example 6, except for changing the temperature at which hydrogen chloride gas was introduced as shown in Table 1. The results obtained are shown in Table 2. REFERENCE EXAMPLES 13 TO 18 Calcination was carried out in the same manner as in Reference Example 6, except for replacing aluminum hydroxide A with the raw material shown in Table 1: The results obtained are shown in Table 2. REFERENCE EXAMPLES 19 TO 30 Aluminum hydroxide A was calcined in the same manner as in Reference Example 6, except for changing the kind and the amount of the seed crystal as shown in Tables 1 and 3. The results obtained are shown in Table 2 and 4. The α-alumina powder obtained in the system using vanadium oxide as a seed crystal (Reference Example 29) had a unique crystal shape as represented by a D/H ratio of 2.0, which is markedly higher as compared with the other systems. Compressive strength of each of α-alumina particle obtained in Reference Examples 22 and 26 was measured. The internal structure of α-alumina obtained in Reference Example 25 was observed with TEM. No defect in the particles was detected. The SEM photograph of the α-alumina powder obtained in Reference Example 19 is shown in FIG.3, and the particle size distribution of this powder is shown in FIG.4. REFERENCE EXAMPLE 31 Aluminum hydroxide A was calcined in the same manner as in Reference Example 6, except that chlorine gas and steam were introduced into the tubular kiln as a gas atmosphere. The calcination conditions and the results obtained are shown in Tables 3 and 4. EXAMPLES 1 AND 2 Aluminum hydroxide A was calcined in the same manner as in Reference Example 6, except for using a shape-controlling agent shown in Table 3 in addition to the seed crystal. The calcination conditions and the results obtained are shown in Tables 3 and 4. REFERENCE EXAMPLES 32 TO 47 Aluminum hydroxide A was calcined in the presence of various metal oxides as a shape-controlling agent. The calcination conditions and the results obtained are shown in Tables 3, 4, 5 and 6. REFERENCE EXAMPLES 48 AND 49 Aluminum hydroxide A was calcined in the same manner as in Reference Example 36 or 33, except for changing the temperature at which the gas atmosphere was introduced. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 50 Calcination was carried out in the same manner as in Reference Example 43, except for replacing aluminum hydroxide A with transition alumina A. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 51 Aluminum hydroxide A was calcined in the same manner as in Reference Example 33, except for using chlorine gas and steam as a gas atmosphere. As a result, the same α-alumina powder as obtained in Reference Example 33 was obtained. The calcination conditions are shown in Tables 5 and 6. REFERENCE EXAMPLE 52 Calcination was carried out in the same manner as in Reference Example 33, except for using alum as a starting material. The resulting α-alumina powder was substantially equal to that obtained in Reference Example 33. The calcination conditions are shown in Tables 5 and 6. REFERENCE EXAMPLE 53 Transition alumina A to which α-alumina A had been added as a seed crystal was calcined in an atmosphere containing hydrogen bromide obtained by decomposition of ammonium bromide. The SEM photograph of the resulting α-alumina powder is shown in FIG.7. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 54 Calcination was carried out in the same manner as in Reference Example 52, except for replacing alum as a raw material with transition alumina A and replacing MgO as a shape-controlling agent with zirconia gel (ZrO 2 ). The SEM photograph of the resulting α-alumina powder is shown in FIG.8. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 55 Calcination was carried out in the same manner as in Reference Example 53, except for replacing transition alumina A as a raw material with aluminum hydroxide A. The resulting α-alumina powder was equal to that obtained in Reference Example 53. The calcination conditions are shown in Tables 5 and 6. REFERENCE EXAMPLE 56 Transition aluminum A was calcined in the same manner as in Reference Example 53, except for using bromine gas and steam as a gas atmosphere. The resulting α-alumina powder was equal to that obtained in Reference Example 53. The calcination conditions are shown in Tables 5 and 6. REFERENCE EXAMPLE 57 Transition alumina A to which α-alumina A had been added as a seed crystal was calcined in a gas atmosphere containing hydrogen iodide obtained by decomposition of ammonium iodide. The SEM photograph of the resulting α-alumina powder is shown in FIG.9. The calcination conditions and the results obtained are shown in Tables 5 and 6. EXAMPLE 3 Transition alumina A was calcined in the same manner as in Reference Example 1, except for replacing α-alumina A as a seed crystal with α-alumina B and further adding zirconia gel (ZrO 2 ) as a shape-controlling agent. The SEM photograph of the resulting α-alumina powder is shown in FIG.10. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 58 Calcination was carried out in the same manner as in Reference Example 57, except for replacing transition aluminum A with aluminum hydroxide A as a starting material. The resulting α-alumina powder was equal to that obtained in Reference Example 57. The calcination conditions are shown in Table 5 and 6. REFERENCE EXAMPLE 59 Transition aluminum A was calcined in the same manner as in Reference Example 57, except for using a gas atmosphere containing iodine gas and steam. The resulting α-alumina powder was equal to that obtained in Reference Example 57. The calcination conditions are shown in Table 5 and 6. REFERENCE EXAMPLE 60 Transition alumina A to which α-alumina A had been added as a seed crystal was calcined in a chlorine gas atmosphere under conditions shown in Tables 5 and 6. The SEM photograph of the resulting α-alumina powder is shown in FIG. 11. The results obtained are shown in Table 6. REFERENCE EXAMPLE 61 Transition alumina A was calcined in the same manner as in Reference Example 60, except for changing the composition of the gas atmosphere and the calcining temperature. The resulting α-alumina powder was equal to that obtained in Reference Example 60. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 62 Transition alumina A was calcined in the same manner as in Reference Example 60, except for replacing α-alumina A as a seed crystal with calcium oxide as a shape-controlling agent. As a result, α-alumina powder having a D/H ration of 3 was obtained. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 63 Aluminum hydroxide A was mixed with 0.1 part by weight of boron oxide (B 2 O 3 ) as a shape-controlling agent by ultrasonication in isopropyl alcohol, and the mixture was calcined in a gas atmosphere consisting of 30% by volume of hydrogen chloride and 70% by volume of nitrogen at 1100°C for 30 minutes. The resulting α-alumina powder had an octahedral shape and a D 90 /D 10 ratio of 2.0. The SEM photograph of the powder is shown in FIG.12. The calcination conditions and the results obtained are shown in Tables 5 and 6. REFERENCE EXAMPLE 64 Aluminum hydroxide A to which 1 part by weight of zirconia gel (ZrO 2 ) as a shape-controlling agent had been added was calcined in the same manner as in Reference Example 63. The resulting α-alumina powder had an eicosahedral shape and a D 90 /D 10 ratio of 2.0. The SEM photograph of the powder is shown in FIG. 13. The calcination conditions and the results obtained are shown in Tables 5 and 6. COMPARATIVE EXAMPLE 1 Aluminum hydroxide A was calcined under conventional conditions, i.e., at 1300° to 1400°C for 1 to 4 hours in air. The resulting α-alumina powder had a D 90 /D 10 ratio of 4.0, an average particle diameter of about 0.7 µm, and an irregular shape. The SEM photograph of the powder is shown in FIG. 14, and the particle size distribution of the powder is shown in FIG. 15. The calcination conditions and the results obtained are shown in Tables 7 and 8. COMPARATIVE EXAMPLE 2 α-Alumina powder was obtained by a hydrothermal process. Compressive strength of particles in the powder was measured. The results obtained are shown in Table 8. COMPARATIVE EXAMPLE 3 α-Alumina powder was obtained by a hydrothermal process. Internal micro structure of particles in the powder were observed with TEM. Many defects in the particles were detected. The result obtained is shown in Table 8. COMPARATIVE EXAMPLES 4 TO 6 α-Alumina powders were obtained by an electrofusion method. The internal micro structure of particles in the powder obtained in Comparative Example 4 was observed with TEM. Many defects in the particles were detected. The tapped density of each of the powders obtained in Comparative Examples 4, 5 and 6 was measured. The results obtained are shown in Table 8. Example No. Gas Feed Temperature (°C) Calcining Temperature (°C) Calcination Time (min) Particle Diameter (µm) D/H Ratio Crystal Habit Ref Ex. 46 800 1100 30 12 0.8 D Ref Ex. 47 800 1100 30 13 0.8 D Ref Ex. 48 400 1100 30 14 1.2 B Ref Ex. 49 1100 1100 30 11 1.2 C, H Ref Ex. 50 800 1100 30 14 1 B Ref Ex. 51 800 1100 30 Ref Ex. 52 800 1100 30 Ref Ex. 53 800 1100 30 0.7 1 E, H Ref Ex. 54 800 1100 30 15 0.7 D Ref Ex. 55 800 1100 30 Ref Ex. 56 800 1100 30 Ref Ex. 57 800 1100 30 0.7 1 F, H Example 3 800 1100 30 3.8 0.6 D Ref Ex. 58 800 1100 30 Ref Ex. 59 800 1100 30 Ref Ex. 60 20 1100 30 0.7 1 F, H Ref Ex. 61 20 1280 30 0.7 1 F, H Ref Ex. 62 1100 1280 30 4 3 I Ref Ex. 63 800 1100 30 14 1.3 B Ref Ex. 64 800 1100 30 18 0.8 D The α-alumina powder of the present invention produced by the process according to the present invention comprises α-alumina particles having a substantially octahedral or eicosahedral shape, a specific structure, and a narrow primary particle size distribution, and therefore has excellent characteristics and is particularly useful as a raw material for abrasives, fillers, sinters or spacers.

Claims: A process for producing α-alumina powder which comprises α-alumina particles having an octahedral or higher polyhedral shape and a hexagonal close-packed lattice having a D/H ratio of from 0.5 to 30, wherein D represents the maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of α-alumina, and H represents the maximum particle diameter perpendicular to the hexagonal lattice plane; comprising the step of calcining in a gas atmosphere selected from (1) a gas atmosphere containing a hydrogen halide, (2) a gas atmosphere containing a component prepared from a halogen and steam, and (3) a gas atmosphere containing a halogen, at least one of transition alumina and a transition alumina precursor capable of being transformed to transition alumina on heating, characterized in that the calcination occurs in the presence of seed crystals and a shape-controlling agent. A process as claimed in Claim 1, wherein said gas atmosphere is said gas atmosphere (1) having a hydrogen halide concentration of not less than 0.1% by volume based on said gas atmosphere. A process as claimed in Claim 1, wherein said gas atmosphere is said gas atmosphere (2) and said component is prepared from not less than 0.1% by volume of a halogen and not less than 0.01% by volume of steam, both percentages being based on said gas atmosphere. A process as claimed in Claim 1, wherein said gas atmosphere is said gas atmosphere (3) having a halogen concentration of not less than 0.1% by volume based on said gas atmosphere. A process as claimed in Claim 1, 2 or 3, wherein said gas atmosphere is said gas atmosphere (1) or (2) and the calcination step is performed at a temperature of from 500° to 1400°C. A process as claimed in Claim 1 or 4, wherein said gas atmosphere is said gas atmosphere (3) and said calcination step is performed at a temperature of from 950° to 1500°C. A process as claimed in any preceding claim, wherein said seed crystals are selected from aluminum compounds, titanium compounds, vanadium compounds, chromium compounds, iron compounds and nickel compounds. A process as claimed in Claim 7, wherein said compounds are oxides, nitrides, oxinitrides, carbides, carbide nitrides, halides or borides. A process as claimed in any preceding claim, wherein said shape-controlling agent is one or more of magnesium, a magnesium compound, calcium, a calcium compound, strontium, a strontium compound, yttrium, a yttrium compound, zirconium, a zirconium compound, vanadium, a vanadium compound, niobium, a niobium compound, molybdenum, a molybdenum compound, copper, a copper compound, zinc, a zinc compound, boron, a boron compound, silicon, a silicon compound, lanthanum, a lanthanum compound, cerium, a cerium compound, neodymium, and a neodymium compound. A process as claimed in Claim 9, wherein said magnesium compound, calcium compound, strontium compound, yttrium compound, zirconium compound, vanadium compound, niobium compound, molybdenum compound, copper compound, zinc compound, boron compound, silicon compound, lanthanum compound, cerium compound or neodymium compound is an oxide, a nitride, an oxinitride, a carbide, a carbide nitride, a halide or a boride. A process as claimed in any preceding claim, wherein said transition alumina precursor is aluminum hydroxide, alum or aluminum sulfate.

Inventors: MOHRI MASAHIDE, SAWABE YOSHINARI, UCHIDA YOSHIO, WATANABE HISASHI, MOHRI, MASAHIDE, SAWABE, YOSHINARI, UCHIDA, YOSHIO, WATANABE, HISASHI
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY LIMITED
Application year: 1994
Main classification: C01F7
IPC codes: C01F7